Gas liquefaction technologies are increasingly also being used for the purposes of transportation and distribution of natural gas, in order to reduce the volume and to allow the gas to be transported at low cost in liquefied form, for example using special tankers.
Large natural gas liquefaction installations use powerful compressors, by which the natural gas is compressed in the course of the liquefaction process. Gas turbines are increasingly being used to drive these compressors and, for example, they use the available natural gas as a fuel. These gas turbines have normally been developed for driving generators for production of electrical power, which is then fed into an electrical grid at a predetermined grid frequency (for example 50 Hz or 60 Hz).
FIG. 1 shows a highly simplified illustration of a compressor section 10 of a known type, in which a gas turbine 12 directly mechanically drives a compressor 11 of a natural gas liquefaction installation via a shaft 19. The compressor 11 inducts gas via a gas inlet 20 and emits compressed gas at a gas outlet 21. In the simplest case, the gas turbine 12 comprises a compressor 13 which inducts and compresses combustion air via an air inlet 16. The compressor 13 may comprise a plurality of partial compressors connected one behind the other, which operate at a rising pressure level and may possibly allow intermediate cooling of the compressed air. The combustion air compressed in the compressor 13 is passed to a combustion chamber 15, into which liquid fuel (for example oil) or gaseous fuel (for example natural gas) is injected via a fuel supply 17, and is burnt with combustion air being consumed.
The hot gases emerging from the combustion chamber 15 are expanded in a downstream turbine 14 with work being carried out, and thus drive the compressor 13 of the gas turbine and the coupled compressor 11 of the natural gas liquefaction installation. The rotation speed of the gas turbine 12 is in this case the same as the rotation speed of the external compressor 11.
The large gas turbine units that are currently standard with powers of more than 50 MW are designed for gas-turbine rotation speeds of 3600 rpm (for a grid frequency of 60 Hz) or 3000 rpm (for a grid frequency of 50 Hz). Precautions therefore have to be taken in order to accelerate the gas turbine together with the compressor to the rated rotation speed, and to dispose of excess power from the gas turbine.
U.S. Pat. No. 5,689,141 discloses a drive system for the compressor of a natural gas liquefaction installation, in which the compressor is driven directly on one side by a gas turbine and is connected on the other side to a synchronous machine. The synchronous machine drives the compressor section during acceleration of the gas turbine, and for this purpose draws power from an electrical grid. When the gas turbine has reached its rotation speed, the synchronous machine operates as a generator, and can convert excess power produced by the gas turbine to electricity, and can feed this back into the electrical grid.
International Patent Application Publication No. WO-A2-2005/047789 discloses a comparable arrangement. As illustrated in FIG. 2, a motor/generator 22 is provided on the common shaft 19 between the gas turbine 12 and the external compressor 11, which is connected to an electrical grid 24 via a variable frequency drive 23. The variable frequency drive 23 ensures soft starting of the compressor section 10′, and feeds excess power at the grid frequency into the electrical grid 24 when the motor/generator 22 is operating as a generator.
The following disadvantages result from the rigid coupling between the turbine rotation speed and the rotation speed of the compressor 11:                stable operation on the external compressor is possible only to a restricted extent.        compressor-independent power control of the power station is impossible.        compressor-independent efficiency optimization of the power station is impossible.        partial load optimization of the power station independently of the grid frequency is impossible.        emission control of the gas turbine is possible only a restricted extent.        
The following disadvantages result from the rigid coupling between the turbine rotation speed and the rotation speed of the compressor for existing installation concepts with components to be newly developed and new installations:                compressors and turbines cannot be designed for the optimum point fixed rotation speed coupling, as is possible in the case of rotation speed independence.        gas and steam turbines which are designed with fixed rotation speed coupling are not necessarily cost-optimum for a desired power since the predetermined rotation speed means that aerodynamic or mechanical design limits impede the optimization process, and these design limits can be better matched with one another by rotation speed variability.        the gas turbines cannot be optimally matched to the variable environmental conditions.        